1. Field of the Invention
The invention relates to the field of wavelength lockers used in conjunction with widely tunable lasers, and more particularly, to an anamorphic prism wavelength locker.
2. Description of the Related Art
There are a variety of semiconductor laser designs in which the wavelength can be tuned by current injection into one or more sections of the laser that have some spectral filtering characteristics. Examples include moderately-tunable devices, such as distributed Bragg reflector (DBR) lasers, and widely-tunable devices, such as sampled grating distributed Bragg reflector (SGDBR) lasers.
In each of these devices, precise control of each of the currents provided to the various sections of the laser is required to achieve a desired output wavelength. The amount of current injected into each section to achieve a given wavelength can vary with both device temperature and aging. For example, over time, as the device ages, changes in leakage current and carrier lifetime in the tuning sections cause the laser output wavelength to drift. This variation in tuning characteristics requires the use of some sort of feedback control system in order for such tunable lasers to be used in applications where precise wavelength control must be maintained, such as dense wavelength division multiplexed (DWDM) communications systems.
One type of feedback control is known as a wavelength locker. A wavelength locker is used for stabilizing the laser output wavelength at one of a plurality of channels. However, prior art wavelength lockers have not been especially compact and thus are not easily integrated into laser device packaging. In addition, prior art wavelength lockers have not been very cost effective. Moreover, prior art wavelength lockers do not provide any type of output beam conditioning, for example, in order to correct irregular output beam cross-sectional configurations generated by edge-emitting diode lasers.
Therefore, there is a need in the art for a wavelength locker that is compact, so that it may be integrated into the laser packaging. In addition, there is a need in the art for a wavelength locker that is cost effective. Moreover, there is a need in the art for a wavelength locker that provides some type of output beam conditioning.
The present invention describes an anamorphic prism wavelength locker for a laser output beam, wherein the anamorphic prisms perform beam splitting for wavelength and power monitoring for the output beam, magnification of the output beam, and circularization of an elliptical output beam. The prisms may comprise an air-spaced pair of prisms or three contacted prisms.
The prisms split off at least two beams from the output beam, wherein a first split-off beam is transmitted to an etalon and a first photo-detector for wavelength monitoring, while a second split-off beam is transmitted to a second photo-detector as a reference and for power monitoring.
The prisms also expand or compress the elliptical output beam width in a first axis, while leaving the elliptical output beam width in a second axis orthogonal to the first axis unchanged, thereby generating a circularized output beam.